1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a method of fabricating a liquid crystal display (LCD) for preventing an electrical short.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) includes switching devices consisting of thin film transistors, each having a gate electrode, a gate insulating film, an active layer, an ohmic contact layer, and source and drain electrodes. Liquid crystal is injected between a lower plate provided with pixel electrodes and an upper plate provided with color filters.
In the LCD, N×M unit pixels (wherein N and M are integers), each of which includes a thin film transistor as a switching device and a pixel electrode that is coupled with the thin film transistor, are vertically and horizontally arranged in a matrix. The pixel electrodes are driven by the thin film transistors to control the liquid crystal's transmission or reflection of incident light.
FIGS. 1A to 1E shows a process of fabricating a conventional LCD.
Referring to FIG. 1A, aluminum (Al) or copper (Cu) is deposited, beneficially by sputtering, on a transparent substrate 11 to form a metal thin film. The metal thin film is then patterned to remain only at a desired portion of the transparent substrate 11 by photolithography, beneficially using a wet method, to form a gate electrode 13 that is electrically connected to a gate line (not shown).
Referring to FIG. 1B, a gate insulating film 15, an active layer 17 and an ohmic contact layer 19 are sequentially formed on the transparent substrate 11 by chemical vapor deposition (CVD) so as to cover the gate electrode 13. The gate insulating film 15 is formed by depositing an insulation material, such as silicon oxide or silicon nitride. The active layer 17 is formed from a deposition of an undoped amorphous silicon or an undoped polycrystalline silicon. The ohmic contact layer 19 is formed from an amorphous silicon, or from a polycrystalline silicon, that is doped with an n-type or p-type impurity at a high concentration.
Desired portions of the ohmic contact layer 19 and the active layer 17 are patterned by photolithography using anisotropic etching so as to expose the gate insulating film 15. At this time, the active layer 17 and the ohmic contact layer 19 remain adjacent the gate electrode 13.
Referring to FIG. 1C, a metal, such as molybdenum (Mo), chrome (Cr), titanium (Ti) or tantalum (Ta), or a molybdenum alloy such as MoW, MoTa or MoNb, is deposited, beneficially by CVD or sputtering, on the gate insulating film 15 and over the ohmic contact layer 19. The deposited metal forms an ohmic contact with the ohmic contact layer 19. Then, the deposited metal is patterned by photolithography to expose the gate insulating film 15, thereby forming source and drain electrodes 21 and 22. At this time, a data line 23 that electrically connects to the source electrode 21 is formed perpendicularly to the gate line (not shown), thus defining a pixel area (not shown). When the source and drain electrodes 21 and 22 are formed, the ohmic contact layer 19 between the source and drain electrodes 21 and 22 is also patterned so as to expose the active layer 17. The active layer between the source and drain electrodes 21 and 22 forms a channel.
Referring to FIG. 1D, a passivation layer 25 that covers the above-mentioned structure is then formed over the transparent substrate 11. The passivation layer 25 is made from an inorganic insulating material, such as silicon nitride or silicon oxide, or from an organic insulation material having a small dielectric constant, such as acrylic organic compound, BCB (β-stagged-divinyl-siloxane benzocyclobutane) or PFCB (perfluorocyclobutane).
Then, the passivation layer 25 is patterned to define a contact hole 26 that exposes the drain electrode 22. A transparent conductive material, such as indium tin oxide (ITO), tin oxide (TO) or indium zinc oxide (IZO), is then deposited on the passivation layer 25 in such a manner as to electrically contact the drain electrode 22 via the contact hole 26. The result is a transparent conductive film 27.
A positive-type photoresist 29 is then coated on the transparent conductive film 27. Ultraviolet rays are then selectively irradiated onto the photoresist 29 using an exposure mask 31 having a shielding part 32 and a transparent part 33. At this time, an exposed area 30 is defined in the photoresist 29. The exposed area forms a high polymer state via the light passing through the transparent part 33 of the exposure mask 31. The exposed area 30 is formed in correspondence with the data line 23, the gate line (not shown), and the thin film transistor.
Referring to FIG. 1E, the photoresist 29 is developed with a developer, such as an aqueous alkali solution. After this, only the unexposed portion of the photoresist 29 remains, while the photoresist 29 in the exposed area 30 is removed to expose the transparent conductive film 27. The remaining photoresist 29 forms a photoresist pattern 35 that acts a mask during the patterning of the exposed portion of the transparent conductive film 27. Patterning is beneficially performed by photolithography using a mixture acid, such as HCl, (COOH)2 or HCl+HNO3, as an etchant liquid. The result is a pixel electrode 37. As shown, the pixel electrode 37 is in electrical contact with the drain area 22. Thereafter, the remaining photoresist pattern 35 is removed.
In the conventional LCD fabricating method described above, the pixel electrode is formed by coating a transparent conductive film with a positive-type photoresist. Next, a pattern for the pixel electrode is produced by exposing and developing the positive-type photoresist so as to expose a portion of the transparent conductive film. That pattern corresponds to a data line, to a gate line, and to a thin film transistor. Finally, the exposed portion of the transparent conductive film is wet etched. However, if a foreign substance is at a portion corresponding to the data line or to the gate line before the photo process, such as before or after the photoresist coating, sufficient light energy is not delivered to the exposed area. The result can be photoresist remaining on a portion of the pixel electrode that corresponds to the data line or to the gate line. Since any remaining photoresist protects the transparent conductive film from being etched, adjacent pixel electrodes can become electrically shorted.